The present invention relates to a filler-containing polytetrafluoroethylene granular powder and a preparation process thereof.
Hitherto, a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder by granulating a mixture of a filler and a PTFE powder with stirring in water was proposed, for example, in JP-B-43-8611, JP-B-44-22619, JP-B-48-37576, JP-B-49-17855, JP-B-56-8044, JP-B-57-18730, etc.
However, by the process for preparation disclosed in each of the above-mentioned patent publications, a filler-containing PTFE granular powder having a small particle size and a narrow particle size distribution could not be obtained.
For that reason, in case of preparing, for example, small articles such as seal rings in the form of a rubber band, thin moldings and molded products having a small surface roughness, there was a problem that complicated and uneconomical steps must be employed, such as step of sieving a filler-containing PTFE granular powder to take out only particles of small size and molding them, or step of cutting the obtained molding.
Also a granular powder having excellent powder flowability cannot be obtained by only pulverizing a filler-containing PTFE granular powder.
In JP-B-60-21694, there was proposed a process for preparing a filler-containing PTFE granular powder by granulating, with stirring in water, a PTFE powder and a filler which has been previously surface-treated with an aminosilane compound in coexistence with a water-insoluble organic liquid and an anionic surfactant. However, an apparent density of the filler-containing PTFE granular powder and tensile strength of molded products obtained therefrom are not fully satisfactory.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by granulating a mixture of a PTFE powder and a filler with stirring in water in the presence of an organic liquid which forms liquid-liquid interface with water and a specific nonionic and/or anionic surfactant.
An object of the present invention is to provide a filler-containing PTFE granular powder having a good processability and a process for preparation thereof. Particularly an object of the present invention is to provide a filler-containing PTFE granular powder which has a large apparent density, a small average particle size, a narrow particle size distribution, a small electrostatic charge and good physical properties such as powder flowability, and gives molded products having excellent physical properties such as elongation and surface smoothness and a high whiteness, and a process for preparation thereof.
The present invention relates to a process for preparing a filler-containing polytetrafluoroethylene (PTFE) granular powder by granulation of a mixture of a polytetrafluoroethylene (PTFE) powder and a filler in water with stirring; characterized in that the granulation is carried out by stirring the mixture in water in the presence of an organic liquid which forms liquid-liquid interface with water and an anionic surfactant and/or nonionic surfactant having a hydrophobic segment comprising a poly(oxyalkylene) unit having 3 or 4 carbon atoms and a hydrophilic segment comprising a poly(oxyethylene) unit.
The above-mentioned process can give a filler-containing PTFE granular powder having an apparent density of not less than 0.7 g/cm3 and an electrostatic charge of not more than 50 V.
The present invention also relates to a filler-containing PTFE granular powder which is characterized in that in case of an apparent density of not less than 0.7 g/cm3 and less than 0.9 g/cm3, an angle of repose is not more than 40 degrees and an electrostatic charge is not more than 10 V; in case of an apparent density of not less than 0.9 g/cm3 and less than 1.0 g/cm3, an angle of repose is not more than 38 degrees and an electrostatic charge is not more than 10 V; and in case of an apparent density of not less than 1.0 g/cm3, an angle of repose is not more than 36 degrees and an electrostatic charge is not more than 10 V and an average particle size is not more than 500 xcexcm.